In forest machines, a harvester head is typically used, which is arranged to perform the felling of a growing tree and the manipulation of the tree after the felling at the forest work place. When the harvester head is fastened to the end of a boom assembly in a forest machine, its task is to grip an upright standing tree and cut it, allow the movement of the tree substantially onto the horizontal plane in a controlled manner and to delimb and cut the tree. The tree trunk is fed through the delimbing means of the harvester head substantially on horizontal plane in such a manner that the travel of the tree is stopped at intervals, wherein the tree trunk is cut at a suitable point by means of sawing. The feeding means typically comprise a pair of feeding wheels between which the tree trunk is positioned. The delimbing means comprise one or several pairs of knives, which are positioned around the tree trunk and surround the tree trunk as well as possible. Typically the entire tree, excluding the relatively thin top part, is manipulated in this way.
The saw assembly performing the sawing is mounted substantially in the shield of a saw box. An advantageous saw assembly comprises a frame, a saw motor, a drive gear, a guide bar, a saw chain, a holder for the guide bar and a member for feeding the guide bar outward and inward. The saw motor is arranged to drive the drive gear and during the sawing process the saw chain rotates around the turnable guide bar, driven by the drive gear. The holder of the guide bar is arranged turnable and the outward and inward feeding member is arranged to move the guide bar. In one embodiment the outward and inward feeding member comprises a hydraulic cylinder installed in the saw box, said hydraulic cylinder comprising a piston rod and connection means for transmitting the movement of a piston rod to the turning guide bar holder. Typically, the saw assembly contains sensor means for determining the position of the guide bar, by means of which the movement of the guide bar is determined and restricted. A known felling and delimbing apparatus is disclosed in the patent publication FI 97111 B. Known saw assemblies are disclosed in the publications U.S. Pat. No. 5,802,946 and WO 98/53666.
The saw assemblies of known apparatuses typically contain a shielding saw box with such a construction that the saw assembly is installed in an installation space reserved for the same. Typically, the saw motor is installed partly outside the saw box, and the moving guide bar is positioned inside the saw box, as well as for example the actuator moving the guide bar. The saw box functions as a stationary, load-bearing structure. A known saw box is disclosed in the publication FI 4158 U.
The guide bar is positioned in the saw box in such a manner that its movement path is unobstructed; there is enough room for the guide bar to bend, and sawdust can move out without obstacles. It is, however, necessary to leave enough room in the saw box so that the parts of the saw assembly can be moved and positioned therein, because otherwise detachment and installation at worksite conditions is impossible. The sawdust produced during sawing must exit in a reliable manner from the saw assembly, wherein the saw box is at the plane of movement of the guide bar either as open as possible, or it is provided with a large number of openings via which the sawdust can exit the saw box. In winter conditions, the exit of snow from the saw box must be ensured.
One dangerous situation that occurs in sawing is the breaking of the saw chain. Situations have occurred in which parts are loosened from the breaking chain, single cuffing teeth, side links or drive links, or other parts that fly into the environment, causing danger situations, especially if people or other working machines are located within the recommended safe area or within the flight path of a loosened part. At worksites where several forest machines, for example a harvester and a forwarder are working, it is not always possible to ensure the minimum safety distance.